Air gap-insulated, double-walled exhaust manifolds have been increasingly used especially in exhaust systems of motor vehicles which together with other air gap-insulated, double-walled exhaust pipes provide for the optimal operation of an emission control device (catalytic converter) arranged downstream of them. They bring about a reduction in the amount of heat released from the exhaust gas to the environment, so that the exhaust gas flows to the emission control device at a higher temperature than in single-walled exhaust manifolds and exhaust pipes. This is significant especially during the warm-up phase of the internal combustion engine, because the catalyst will thus rapidly reach its working temperature.
Prior-art double-walled exhaust manifolds have an outer pipe and a one-part or multipart inner pipe, which are shaped parts made of sheet metal in a half-shell design. After the pressing of the sheet metal, the blanks are assembled, and the outer half shells of the outer pipe are welded together. Such a manufacturing process is relatively expensive and requires many individual parts, along with an increased material consumption.